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Currently, there are three AOAC Official Methods available for the determination of the free and
unreacted urea in liquid and water-soluble urea containing fertilizers.
The AOAC Official Method 959.03 is the most well-known and the oldest method for urea determination
in fertilizers. This method, which was introduced in 1959, is based on the quantitative hydrolysis of urea
to ammonia by means of urease enzyme. The amount of ammonia generated by hydrolysis is
subsequently determined and reported as equivalent urea or urea N by a simple titration. With most
simple and conventional (i.e. non-SRN-containing) liquid fertilizers, the urease method works very well.
Urea-based methods are well accepted and usually quite suitable for urea determinations in most urea-
containing fertilizers, however, they have been found to demonstrate significant problems, particularly
with incomplete recovery of urea, when applied to such determinations in Urea-Formaldehyde (UF)
fertilizer solutions. Comparisons of the HPLC methods, particularly the AOAC Official Method 2003.14
with results from the urease method, frequently produced results that were not in agreement with each
other. Further study also showed that urea results from the urease methods often were not
reproducible. Thus a controlled study, which is the principle subject of this article, was carried out to
confirm and elaborate upon these observations, and to suggest some likely reasons for the reduced
effectiveness of the urease method with UF fertilizers.
Experimental
Materials
Twelve of the most commonly available commercial liquid urea-based fertilizers were selected for the
determination of their free urea contents using the three aforementioned methods. These commercial
samples were:
1.
LF3060 (30-0-0), Koch Agronomic Services, LLC, 4111 E. 37
th
St. N, Wichita, KS 67220, USA
2.
CoRoN (28-0-0), Helena Chemical Company, 225 Schilling Blvd., Collierville, TN 38017
3.
Arclin 28-0-0, Arclin Resins, Research & Technology, 4754 28
th
St., Springfield, OR 97477
4.
Greenfeed 27-0-0 (light Green liquid), Plant Food Company, 38 Hightstown-Cranbury Station
Road, Cranbury, NJ 08512
5.
Greenfeed 27-0-0 (Dark Green liquid), Plant Food Company, 38 Hightstown-Cranbury Station
Road, Cranbury, NJ 08512
6.
NDemand 30-0-0, Wilbur-Ellis Agribusiness, 3300 S. Parker Road, Suite 500, Aurora, CO 80014,
USA
7.
NDemand 30L, Wilbur-Ellis Agribusiness, 3300 S. Parker Road, Suite 500, Aurora, CO 80014, USA
8.
Gradual N (30-0-0), Winfield Solutions, LLC, 1080 County Road West, Shoreview, MN 55126
9.
N-28 clear Fertilizer (28-0-0), Advachem, Route de Wallonie, Darse d’Hautrage, BE 7334
Hautrage
10.
N-Sure (28-0-0), Tessenderlo Kerley, Inc., 2255 N. 44
th
St., Suite 300, Phoenix, AZ
11.
Trisert NB (26-0-0), Tessenderlo Kerley, Inc., 2255 N. 44
th
St., Suite 300, Phoenix, AZ
12.
Formolene Plus (30-0-0), Tessenderlo Kerley, Inc., 2255 N. 44
th
St., Suite 300, Phoenix, AZ